There are three major types of brushless electric machines available for the electric vehicle (HV) and hybrid electric vehicle (HEV) drive systems. These are the induction machine, the PM machine, and the switched-reluctance machine.
Permanent magnet (PM) machines have been recognized for having a high power density characteristic. A PM rotor does not generate copper losses. One drawback of the PM motor for the above-mentioned application is that the variation of the air gap flux produced by the PM rotor is limited, and therefore, a sophisticated approach is required for high speed, field weakening operation. Another constraint is that inductance is low, which means that current ripple must be controlled.
It is understood by those skilled in the art that a PM electric machine has the property of high efficiency and high power density, however, the air gap flux density of a PM machine is limited by the PM material, which is normally about 0.8 Teslas and below. A PM machine cannot operate at an air gap flux density as high as desired. When the PM motor needs to have a weaker field with a reasonably good current waveform for high-speed operation, a sophisticated power electronics inverter is required.
Hsu, U.S. Pat. No. 6,573,634, issued Jun. 3, 2003, and entitled “Method and Machine for High Strength Undiffused Brushless Operation” discloses an axial gap PM machine for higher strength, undiffused operation.
In many applications, a radial gap machine is preferred. When considering a radial gap configuration for undiffused, high strength operation, several problems have to be overcome. It is desirable to provide a compact design with a shape similar to a conventional radial gap machine. It is not apparent how to arrange the PM material so as to control diffusion between poles of opposite polarity. It is also not apparent how to design the auxiliary field coils so as to complete a magnetic circuit through the rotor.
The electric machine of U.S. patent application Ser. No. 10/668,586 filed Sep. 23, 2003, simplifies the field weakening control requirements. The compactness of the motor is a very important requirement for a vehicle or other applications. In this prior application, the rotor was provided with extension and a ring structure for facing the field winding assemblies across an axial air gap. The present invention continues the ability to enhance and weaken flux in the primary air gap, while improving the construction of the rotor.